A recent electronic device such as of a semiconductor continues to increase its performance. On the other hand, heat generated therefrom is also increasing. Since an electronic device such as of a semiconductor has an application limit of temperature, the generated heat must be dissipated or diffused rapidly and efficiently. Accordingly, a material having a high thermal conductivity is joined to an electronic device such as of a semiconductor to stabilize its performance. This material is generally called heat-dissipating material.
The properties required for the heat-dissipating material include a high thermal conductivity and non-occurrence of crack and separation at a joint surface even if it is joined to an electronic device such as of a semiconductor.
Copper is known as a metal having a superior thermal conductivity. However, its thermal expansion coefficient is greater than 10×10−6/K and 2-3 times greater than that of a semiconductor (silicon: 3.5×10−6/K, gallium arsenide: 5.7×10−6/K). If such copper is joined to a semiconductor and heat is applied to the resulting combination, a large thermal stress is produced at a joint surface due to a difference in thermal expansion coefficient between them to result in the occurrence of crack or separation, which has been a problem.
In the effort to adjust a thermal expansion coefficient, copper has been used in combination with molybdenum or tungsten, i.e., in the form of copper molybdenum or copper tungsten. Their thermal expansion coefficients are not greater than 6×10−6/K and their thermal conductivities are 180-220 W(m·K).
However, a semiconductor will very likely continue to increase its performance. There accordingly is a need for a material which releases heat originating from a semiconductor more efficiently than before.
Since a graphite material is light-weight and low in thermal expansion coefficient due to its basic properties, it has an advantage in being hard to separate even if joined to a silicon semiconductor.
A graphite material consisting basically of graphite particles is reported to exhibit the highest thermal conductivity of 220 W(m·K).
If the thermal conductivity of a graphite material is to be increased, significant growth of its crystal must be accomplished. A method is known for achieving significant crystal growth by adding a catalyst metal to a graphite material and then subjecting the resultant to a heat treatment. However, the use of natural or artificial graphite as a raw aggregate material leads to the absence of a network which links graphite particles to each other. As a result, heat transfer of the obtained graphite product is rendered insufficient. Also, in the case where a binder is used, no shrinkage of the aggregate material occurs during the heat treatment. This increases the occurrence of pores to result in the difficulty to obtain a sufficient thermal conductivity (Patent Literature 1).
In a method wherein a metal is added to a graphite precursor, a heat treatment activates the added metal to promote growth of a graphite crystal so that crystallization of a graphite material is accelerated. Silicon carbide or the like is used for such a catalyst metal. However, the resulting thermal conductivity is still low in value, about 200 W(m·K).    Patent Literature 1: Japanese Patent Laid-Open No. Sho 60-246216